1. Field of the Invention
The present invention relates, in general, to computerized tomography and, specifically, to three-dimensional models generated from computerized tomographic generated data.
2. Background Description
Three-dimensional imaging was pioneered in the late 970's and early 1980's and represents an advance in computerized imaging, which, as one of its medical applications, allows three-dimensional data routinely utilized in CAT (Computerized Axial Tomography) scanning or other imaging technologies (MRI, ultrasound, etc.) to be assembled into a three-dimensional image for display on a CRT screen or converted to a film record.
In three-dimensional computerized axial tomography, an X-ray source is collimated to form a fan beam which is transmitted through an imaged object to an X-ray detector array located in the same imaging plane as the X-ray source. The detected intensity of the transmitted radiation in the X-ray fan beam is dependent on attenuation of the X-ray beam by the imaged object. The intensity of the transmitted radiation is detected by the X-ray detector array.
Typically, the X-ray source and the detector array are mounted in a gantry having a through aperture through which the imaged object is moved. The X-ray source and detector array can be mounted for 360.degree. rotation within the gantry around the imaged object. During each 360.degree. rotation, a two-dimensional cross-sectional slice of the imaged object is generated by the detector array. Each cross-sectional slice is formed of a number of rows and columns of voxels. Each voxel has an anisotropic shape, i.e., generally rectangular, with typical dimensions of 1.5 mm.times.0.35 mm.times.0.35 mm for a high resolution study. The signal intensity of each voxel is represented by a digital number corresponding to the detected signal intensity, i.e., hounsfield unit, for each voxel.
Since a complete scan of the imaged object is made by moving the imaged object through the gantry, a plurality of contiguous, substantially parallel, cross-sectional tomographic slices are generated. The plurality of two-dimensional slices are reviewed by a radiologist, either on a display or on a film record, who mentally integrates the two-dimensional data into a three-dimensional image of the imaged object. Synthesized three-dimensional images are also generated from two-dimensional tomographic data. Such synthesis extracts a mathematical description of the imaged object from the plurality of two-dimensional tomographic data slices before reconstructing a three-dimensional image from the mathematical description using standard operator-dependent thresholding algorithms.
Such three-dimensional images, however, are subject to low accuracy with respect to depicting certain surfaces in an imaged object due to partial volume averaging principles used in generating a digital number corresponding to the signal intensity of each voxel. Partial volume averaging comes into play when a voxel contains a representation of more than one type of substance, i.e., tissue, bone, air, etc. Such a voxel is represented by an average value of these substances based on the percentage occupancy of the voxel by the different substances as well as certain imaging characteristics of tissues (i.e., linear attenuation coefficient in X-ray computerized tomography). For example, a region of low intensity in the imaged object which is parallel to the scan or image plane would occupy only a small portion of the voxels in a particular cross-sectional slice. Such a region will not effect the voxel density as significantly as when the region of low density occupies a greater proportion of individual voxels as might occur when the region of low density is perpendicular to the scan or image plane.
Such perpendicular or parallel orientation of regions of an imaged object results from the orientation of the scan angles or scan planes normally used in computerized tomography. At the present, only a transaxial scan plane, i.e., a scan plane which is oriented perpendicular to the longitudinal axis of the scanner/imaged object, is routinely utilized for three-dimensional reconstructions. This is the consequence of the earlier generation of three-dimensional reconstruction computer programs which could only use transaxial two-dimensional computerized tomography data. Thus, the use of transaxial oriented scan planes does not alone provide the desired accuracy for computerized tomography data to generate three-dimensional images or models of an imaged object.
Varying the scan angle has been proposed to better visualize certain structures within an imaged object. In this process, the scan plane for a number of continuous, tomographic cross-sectional slices is tilted or oriented at a predetermined angle other than 90.degree. to the longitudinal axis of the scanner/imaged object. However, this results in a geometrically distorted image which must be corrected in the reconstruction algorithms to remove the distortion. This technique utilizes a non-perpendicular scan plane angle to generated tomographic data cross-sectional slices with no indication of whether the predetermined angle is optimum for visualizing certain regions of the imaged object.
Thus, it would be desirable to provide a three-dimensional model generation process utilizing multiple angle scan planes which are non-perpendicular to the longitudinal axis of the scanner/imaged object to optimize the generation of three-dimensional models from two-dimensional tomographic data.